Negative Electron Affinity Detector (NEADs)
In the late 1970s, NEADs were discussed as 'a most promising new
technology for X-ray
detection'. A NEAD, or negative electron affinity
detector, is a device with highly desirable properties: high spatial
resolution, high quantum efficiency, and moderate
energy
resolution. In the current literature, however, you will rarely even find
a reference to a NEAD.
Semiconducting compounds composed of elements from the 3rd and 5th columns
of the periodic table (GaAs, for instance) can be activated to a state of
negative electron affinity by treatment of the surface with cesium and
oxygen. That is to say, the top of the conduction band in the bulk can be
made to lie above the vacuum level. Conduction band electrons can
therefore escape from the surface when an X-ray is absorbed, even if the
photon interaction takes place deep within the material. This is what makes
the quantum detection efficiency be near 1. The moderate energy resolution
follows from the fact that the most probable number of emitted
photoelectrons increases monotonically with energy.
Although heavily investigated and modeled at the Smithsonian Astrophysical
Observatory, a position-sensitive NEAD was never demonstrated. The
stumbling block seems to have been the need to maintain the NEA surface in
an extremely high vacuum (10-10 Torr) in order to prevent contamination
which would kill the negative electron affinity property. For the large
area detectors needed in X-ray astronomy, this was indeed a very hard state
to attain and keep... at least at the time of the 1970s and 1980s when they
were being investigated. Maybe NEADs will become possible
again as technology continues to develop.
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